This invention concerns improvements in or relating to artificial eye lenses and relates more particularly, but not exclusively, to bi-focal artificial eye lenses. By the term "artificial eye lens" is meant an artificial lens which in use is disposed against the eye or within the eye. Thus one particular type of artificial eye lens is a contact lens which is used against the eye to assist the action of the natural eye lens, and another is an implant lens which is inserted in the eye to replace the natural eye lens.
There have been various prior proposals for contact lenses including bi-focal contact lenses. Generally a bi-focal lens is required to provide a certain power for far vision and a different, usually greater (more positive), power for near vision, the additional power for near vision sometimes being referred to as a "near-add" value which is usually expressed in dioptres. Normally the bi-focal effect is achieved by the use of different curvatures and/or materials of different refractive indices for different parts of the lens, so as to provide the required different powers in the respective different parts, which are often referred to as the far and near zones. Thus the user sees far objects by means of light transmitted through the far zone part of the lens and near objects by means of light transmitted through the near zone part of the lens. It has also been proposed (in U.S. Pat. No. 3,339,997) effectively to utilize the chromatic aberration of the eye and to provide far and near zones which transmit different wavelengths of light. Specifically different parts of the lens providing the far and near zones are made of differently coloured filtering material.
This proposal claims the advantage that the same curvature can be used for the far and near zones, the effective power difference being provided by the different wavelengths transmitted. However, the amount of power difference which can be achieved in this manner is limited. In realistic terms near-add values of up to about one dioptre can be provided by selection of appropriate colours and, although in theory greater values are possible by use of violet for the near zone and red for the far zone, there is the practical problem that the colours are darker and less light enters the eye. Further, such a lens still requires distinct parts providing the near and far zones.
Implant or intra-ocular lenses are designed to be inserted within the eye by an ophthalmological surgeon after the removal of the natural lens for reasons of its pathological conditions such as cataract. Whereas the natural lens may be deformed by the ciliary muscle to effect accommodation, that is the ability of the eye to focus on objects at different distances, the implant lens is both rigid and not connected to the ciliary muscle. The eye thus treated, while giving better vision than previously, is totally lacking in accommodation, a situation that applies to the natural lens in later life due to the hardening of the natural lens, a condition known as presbyopia.